3.8L auto won't shift into 3rd

First time posting in the v6 forums. My sisters 98 wont shift into 3rd. Does it just need a rebuild or could it be a sensor or computer problem. No check engine light. Anything I can do? she doesnt really have the money for the rebuild right now.

I have a 94 5.0 AODE, is her trans exactly the same as mine?

Thank you